in my opinion there's a thing that's even better:
in every scuba-diving-shop you get a hook which is made for cutting lines and ropes.
they are lighter, smaller and cheaper than knives.
a regular knive made for being used in the sea needs a lot of care and costs about $30+

I have used z knives for about ten years. Mainly for diving but the last three for kitesurfing with no problems. They work well on kite lines but for bridle lines you will probably need a serrated knife.